WALL-NORMAL-MICROJET-BASED DRAG REDUCTION OF HIGH REYNOLDS NUMBER TURBULENT BOUNDARY LAYERS

This work aims to develop a technology to reduce skin friction in the turbulent boundary layer of a high-speed train, which is both effective and efficient even at high friction Reynolds numbers (Reτ). To this end, a high-resolution force balance has been innovatively developed to measure accurately the drag reduction (DR) over the control area. The arrays of wall-normal microjets through spanwise slits are deployed, which have been demonstrated to be able to yield a significant DR in spite of additional drag due to slit-associated surface roughness. Experiments were carried out with a Reτ range from 1000 to 18,000. Results obtained indicate that the maximum spatially-averaged DR over the control area may shoot beyond 70%, though the maximum net energy saving occurs when the DR is only 30-40%. This net energy saving grows with increasing free-stream velocity; its maximum may currently reach 25% at 40 m/s. Hotwire and flow visualization data analyses indicate that the microjet blowing lifts up the streamwise vortices in the TBL. Furthermore, the microjets are of zero streamwise momentum, acting to decrease the near-wall streamwise velocity gradient. Both contribute to the appearance of local flow relaminarization and the significant DR.